Brown spiders in the genus Loxosceles are notorious for bites that cause severe dermonecrotic lesions. Sphingomyelinase D has been identified within the brown recluse, L. reclusa, as a major factor in the formation of these lesions. This enzyme is not known to exist anywhere else in the animal kingdom. However, it is expressed as a toxin in a few bacterial species. There are roughly 100 species of Loxosceles distributed worldwide. The venoms of only a few species have been described, but there is clinical evidence that bites from many widely geographically distributed species are capable of causing necrosis. The range of species that have this enzyme is not known. This proposed research has three main objectives: (1) to determine the phylogenetic distribution of sphingomyelinase D activity within the genus Loxosceles, and among close relatives, (2) to study the molecular evolution of this enzyme to distinguish among competing hypothesis of the mechanism of origin of sphingomyelinase D in this lineage, and (3) to compare the effects of venom on various insect prey (their presumed primary targets of venom in it's natural biological role) between Loxosceles and close relatives that do not have sphingomyelinase D. Together, achieving these objectives will inform our understanding of the timing and circumstances of evolutionary origin, and the distribution and diversity of the enzyme sphingomyelinase D, a rare enzyme that is toxic to humans.